Packet-Scheduling and Shared Channel Transmission
In wireless communication systems employing packet-scheduling, at least part of the air-interface resources are assigned dynamically to different users (mobile stations—MS or user equipments—UE). Those dynamically allocated resources are typically mapped to at least one Physical Uplink or Downlink Shared CHannel (PUSCH or PDSCH). A PUSCH or PDSCH may for example have one of the following configurations:
One or multiple codes in a CDMA (Code Division Multiple Access) system are dynamically shared between multiple MS.
One or multiple subcarriers (subbands) in an OFDMA (Orthogonal Frequency Division Multiple Access) system are dynamically shared between multiple MS.
Combinations of the above in an OFCDMA (Orthogonal Frequency Code Division Multiplex Access) or a MC-CDMA (Multi Carrier-Code Division Multiple Access) system are dynamically shared between multiple MS.
FIG. 1 shows a packet-scheduling system on a shared channel for systems with a single shared data channel. A sub-frame (also referred to as a time slot) reflects the smallest interval at which the scheduler (e.g., the Physical Layer or MAC Layer Scheduler) performs the dynamic resource allocation (DRA). In FIG. 1, a TTI (transmission time interval) equal to one sub-frame is assumed. It should be born noted that generally a TTI may also span over multiple sub-frames.
Further, the smallest unit of radio resources (also referred to as a resource block or resource unit), which can be allocated in OFDM systems, is typically defined by one sub-frame in time domain and by one subcarrier/subband in the frequency domain. Similarly, in a CDMA system this smallest unit of radio resources is defined by a sub-frame in the time domain and a code in the code domain.
In OFCDMA or MC-CDMA systems, this smallest unit is defined by one sub-frame in time domain, by one subcarrier/subband in the frequency domain and one code in the code domain. Note that dynamic resource allocation may be performed in time domain and in code/frequency domain.
The main benefits of packet-scheduling are the multi-user diversity gain by time domain scheduling (TDS) and dynamic user rate adaptation.
Assuming that the channel conditions of the users change over time due to fast (and slow) fading, at a given time instant the scheduler can assign available resources (codes in case of CDMA, subcarriers/subbands in case of OFDMA) to users having good channel conditions in time domain scheduling.
Specifics of DRA and Shared Channel Transmission in OFDMA
Additionally to exploiting multi-user diversity in time domain by Time Domain Scheduling (TDS), in OFDMA multi-user diversity can also be exploited in frequency domain by Frequency Domain Scheduling (FDS). This is because the OFDM signal is in frequency domain constructed out of multiple narrowband subcarriers (typically grouped into subbands), which can be assigned dynamically to different users. By this, the frequency selective channel properties due to multi-path propagation can be exploited to schedule users on frequencies (subcarriers/subbands) on which they have a good channel quality (multi-user diversity in frequency domain).
For practical reasons in an OFDMA system the bandwidth is divided into multiple subbands, which consist out of multiple subcarriers. I.e., the smallest unit on which a user may be allocated would have a bandwidth of one subband and a duration of one slot or one sub-frame (which may correspond to one or multiple OFDM symbols), which is denoted as a resource block (RB). Typically, a subband consists of consecutive subcarriers. However, in some case it is desired to form a subband out of distributed non-consecutive subcarriers. A scheduler may also allocate a user over multiple consecutive or non-consecutive subbands and/or sub-frames.
For the 3GPP Long Term Evolution (3GPP TR 25.814: “Physical Layer Aspects for Evolved UTRA”, Release 7, v. 7.1.0, October 2006 —available at http://www.3gpp.org and incorporated herein by reference), a 10 MHz system (normal cyclic prefix) may consist out of 600 subcarriers with a subcarrier spacing of 15 kHz. The 600 subcarriers may then be grouped into 50 subbands (a 12 adjacent subcarriers), each subband occupying a bandwidth of 180 kHz. Assuming, that a slot has a duration of 0.5 ms, a resource block (RB) spans over 180 kHz and 0.5 ms according to this example.
In order to exploit multi-user diversity and to achieve scheduling gain in frequency domain, the data for a given user should be allocated on resource blocks on which the users have a good channel condition. Typically, those resource blocks are close to each other and therefore, this transmission mode is in also denoted as localized mode (LM).
An example for a localized mode channel structure is shown in FIG. 2. In this example neighboring resource blocks are assigned to four mobile stations (MS1 to MS4) in the time domain and frequency domain. Each resource block consists of a portion for carrying Layer 1 and/or Layer 2 control signaling (L1/L2 control signaling) and a portion carrying the user data for the mobile stations.
Alternatively, the users may be allocated in a distributed mode (DM) as shown in FIG. 3. In this configuration, a user (mobile station) is allocated on multiple resource blocks, which are distributed over a range of resource blocks. In distributed mode a number of different implementation options are possible. In the example shown in FIG. 3, a pair of users (MSs ½ and MSs ¾) shares the same resource blocks. Several further possible exemplary implementation options may be found in 3GPP RAN WG#1 Tdoc. R1-062089, “Comparison between RB-level and Sub-carrier-level Distributed Transmission for Shared Data Channel in E-UTRA Downlink”, August 2006 (available at http://www.3gpp.org and incorporated herein by reference).
It should be noted, that multiplexing of localized mode and distributed mode within a sub-frame is possible, where the amount of resources (RBs) allocated to localized mode and distributed mode may be fixed, semi-static (constant for tens/hundreds of sub-frames) or even dynamic (different from sub-frame to sub-frame).
In localized mode as well as in distributed mode in—a given sub-frame—one or multiple data blocks (which are inter alia referred to as transport-blocks) may be allocated separately to the same user (mobile station) on different resource blocks, which may or may not belong to the same service or Automatic Repeat reQuest (ARQ) process. Logically, this can be understood as allocating different users.
L1/L2 Control Signaling
In order to provide sufficient side information to correctly receive or transmit data in systems employing packet scheduling, so-called L1/L2 control signaling (Physical Downlink Control CHannel-PDCCH) needs to be transmitted. Typical operation mechanisms for downlink and uplink data transmission are discussed below.
Downlink Data Transmission
Along with the downlink packet data transmission, in existing implementations using a shared downlink channel, such as 3GPP-based High Speed Data Packet Access (HSDPA), L1/L2 control signaling is typically transmitted on a separate physical (control) channel.
This L1/L2 control signaling typically contains information on the physical resource(s) on which the downlink data is transmitted (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the mobile station (receiver) to identify the resources on which the data is transmitted. Another parameter in the control signaling is the transport format used for the transmission of the downlink data.
Typically, there are several possibilities to indicate the transport format. For example, the transport block size of the data (payload size, information bits size), the Modulation and Coding Scheme (MCS) level, the Spectral Efficiency, the code rate, etc. may be signaled to indicate the transport format (TF). This information (usually together with the resource allocation) allows the mobile station (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. In some cases the modulation scheme maybe signaled explicitly.
In addition, in systems employing Hybrid Automatic Repeat reQuest (HARQ), HARQ information may also form part of the L1/L2 signaling. This HARQ information typically indicates the HARQ process number, which allows the mobile station to identify the Hybrid ARQ process on which the data is mapped, the sequence number or new data indicator, allowing the mobile station to identify if the transmission is a new packet or a retransmitted packet and a redundancy and/or constellation version. The redundancy version and/or constellation version tells the mobile station, which Hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation)
A further parameter in the HARQ information is typically the UE Identity (UE ID) for identifying the mobile station to receive the L1/L2 control signaling. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other mobile stations to read this information.
The table below (Table 1) illustrates an example of a L1/L2 control channel signal structure for downlink scheduling as known from 3GPP TR 25.814 (see section 7.1.1.2.3 —FFS=for further study):
TABLE 1FieldSizeCommentCat. 1ID (UE or group specific)[8-9]Indicates the UE (or (Resourcegroup of UEs) for indication)which the data transmission is intendedResource assignmentFFSIndicates which (virtual) resource units (and layers in case of multi-layertransmission) the UE(s) shall demodulate.Duration of assignment2-3The duration for which the assignment is valid, could also be used to control the TTI or persistent scheduling.Cat. 2Multi-antenna relatedFFSContent depends on (transportinformationthe MIMO/format)beamforming schemes selected.Modulation scheme2QPSK, 16QAM, 64QAM. In case of multi-layertransmission, multiple instances may be required.Payload size6Interpretation could depend on e.g., modulation scheme and the number ofassigned resource units (c.f., HSDPA).In case of multi-layertransmission, multiple instances may be required.Cat. 3IfHybrid 3Indicates the hybrid (HARQ)asynchronousARQARQ process the hybrid ARQ process current transmission is adoptednumberis addressing.Redundancy2To support versionincrementalredundancy.New data1To handle soft buffer indicatorclearing.IfRetrans-2Used to derive synchronousmissionredundancy version hybrid ARQ sequence(to support is adoptednumberincremental redundancy)and ‘new data indicator’ (to handle soft buffer clearing).
Uplink Data Transmission
Similarly, also for uplink transmissions, L1/L2 signaling is provided on the downlink to the transmitters in order to inform them on the parameters for the uplink transmission. Essentially, the L1/L2 control channel signal is partly similar to the one for downlink transmissions. It typically indicates the physical resource(s) on which the UE should transmit the data (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA) and a transport format the mobile station should use for uplink transmission. Further, the L1/L2 control information may also comprise Hybrid ARQ information, indicating the HARQ process number, the sequence number or new data indicator, and further the redundancy and/or constellation version. In addition, there may be a UE Identity (UE ID) comprised in the control signaling.
Variants
There are several different flavors how to exactly transmit the information pieces mentioned above. Moreover, the L1/L2 control information may also contain additional information or may omit some of the information. For example, the HARQ process number may not be needed in case of using no or a synchronous HARQ protocol. Similarly, the redundancy and/or constellation version may not be needed, if for example Chase Combining is used (i.e., always the same redundancy and/or constellation version is transmitted) or if the sequence of redundancy and/or constellation versions is pre-defined.
Another variant may be to additionally include power control information in the control signaling or MIMO related control information, such as, e.g., pre-coding information. In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.
In case of uplink data transmission, part or all of the information listed above may be signaled on uplink, instead of on the downlink. For example, the base station may only define the physical resource(s) on which a given mobile station shall transmit,. Accordingly, the mobile station may select and signal the transport format, modulation scheme and/or HARQ parameters on the uplink. Which parts of the L1/L2 control information is signaled on the uplink and which proportion is signaled on the downlink is typically a design issue and depends on the view how much control should be carried out by the network and how much autonomy should be left to the mobile station.
Another, more recent suggestion of a L1/L2 control signaling structure for uplink and downlink transmission may be found in 3GPP TSG-RAN WG1  #50 Tdoc. R1-073870, “Notes from offline discussions on PDCCH contents”, August 2007, available at http://www.3gpp.org and incorporated herein by reference.
As indicated above, L1/L2 control signaling has been defied for systems that are already deployed to in different countries, such as for example, 3GPP HSDPA. For details on 3GPP HSDPA it is therefore referred to 3GPP TS 25.308, “High Speed Downlink Packet Access (HSDPA); Overall description; Stage 2”, version 7.4.0, September 2007 (available at http://www.3gpp.org) and Harri Holma and Antti Toskala, “WCDMA for UMTS, Radio Access For Third Generation Mobile Communications”, Third Edition, John Wiley & Sons, Ltd., 2004, chapters 11.1 to 11.5, for further reading.
L1/L2 control signaling reduction techniques
For scheduling (delay-sensitive) services with small data packets, such as, e.g., VoIP (Voice over IP) or gaming, the downlink L1/L2 control signaling can be quite significant if each small data packet needs to be signaled. In a 5 MHz 3GPP LTE system, up to 400 VoIP users can be supported as has been shown in 3GPP TSG-RAN WG1 Meeting #46 Tdoc. R1-062179, “VoIP System Performance for E-UTRA Downlink—Additional Results”, (available at    http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1_46/Docs/). This results in roughly 10 VoIP packets on the uplink and 10 VoIP packet on the downlink within a sub-frame, which requires 20 L1/L2 control channels (10 for uplink data transmission and 10 for downlink data transmission). Assuming that the payload size of an L1/L2 control channel carrying an uplink allocation is 35-45 bits and the payload size of an L1/L2 control channel carrying an downlink allocation is approximately 35-50 bits, this results in an downlink L1/L2 control channel overhead of roughly 25-34% (assuming QPSK rate ⅓ transmission of the L1/L2 control channels). This overhead is significantly larger than for other services (e.g., FTP, HTTP, audio/video streaming), where the data can be transmitted in large packets (assumed downlink L1/L2 control channel overhead in this case is approximately 8-12%). Therefore, within the 3GPP LTE standardization the several reduction techniques for services with small data packets are investigated. In the following two investigated schemes that are discussed by the 3GPP are briefly explained:
One scheme discussed is based on a grouping of users (e.g., in similar radio conditions) Examples of this scheme are described in the parallel European patent application no. EP 06009854.8, “RESOURCE RESERVATION FOR USERS IN A MOBILE COMMUNICATION SYSTEM” or in 3GPP TSG-RAN-WG2 Meeting #57 Tdoc. R2-070758, “Scheduling for downlink” (available at    http://www.3gpp.org/ftp/tsg_ran/WG2RL2/TSGR2_57/Documents/), both documents being incorporated herein by reference. In this scheme, a single downlink L1/L2 control channel with a special “group format” is used. This causes that less “group format” downlink L1/L2 control channels are required to be transmitted than “normal” L1/L2 control channels. Although the payload size of the “group format” L1/L2 control channels is larger than that of the “normal” L1/L2 control channel, a net saving in L1/L2 control signaling overhead is expected.
Another exemplary scheme is based on the use of a persistent allocation downlink resources and using with blind detection. Examples of this scheme are described in the parallel European patent application no. EP 06009854.8 mentioned above or in 3GPP TSG-RAN WG2 Meeting #56bis R2-070272, “Signalling optimized DL scheduling for LTE” (available at    http://www.3gpp.org/ftp/tsg_ran/WG2_RL2/TSGR2_56bis/Documents/and incorporated herein by reference). In this exemplary scheme, a certain set of resource blocks and/or subframes (e.g., a certain time-frequency window) and possibly a certain set of transport formats is pre-configured and the UE tries to blindly decode the possibly transmitted packet on the pre-configured resources with the pre-configured set of transport formats. For the initial transmission of a packet the downlink L1/L2 control channel is omitted, whereas retransmissions are allocated by the downlink L1/L2 control channel. Assuming that the packet error rate for the first transmission of a packet is considerably low, L1/L2 control signaling overhead is reduced, e.g., for a 10% packet error rate for the first transmission, the L1/L2 control signaling overhead can be roughly reduced by 90%. Typically, in such a scheme, the L1/L2 control signaling transmitted with the retransmission carries information about the initial transmission (e.g., information about to a subframe at which the initial transmission took place, information about the resource block(s) on which the initial transmission has been allocated and/or information about the transport format).
It is therefore desirable to reduce of the mobile station (UE) complexity with respect to decoding the downlink L1/L2 control channels. It is further desirable to achieve an additional reduction of the downlink L1/L2 control signaling overhead and increase in signaling efficiency. Additionally, it may be appreciated by those skilled in the art to implement a simple and less complex downlink L1/L2 control channel structure.